The present invention relates to a method and an apparatus for monitoring an activity of partial electrical discharges in an electrical apparatus powered with direct voltage.
More in general, the technical field of the present invention is that of the diagnostics of electrical systems (in particular high voltage systems), by measuring/processing partial electrical discharges.
It should be noted that a partial discharge is an electrical discharge that involves a limited portion of an insulator of an electrical system, therefore it does not cause the immediate failure of the system, but its progressive degradation. Thus, partial discharges have, by their nature, a development that is substantially limited to a defect of the insulating system.
In this light, diagnostic techniques based on the measurement and interpretation of partial discharges are among the most promising ones and they are widely studied within scientific research, because the assessment of signals relating to partial discharges enables to investigate the nature of the defects of the insulating system in which the discharges take place and the position of the defects within the insulating system. In this light, it should be noted that measuring and monitoring partial discharges is a consolidated practice with regard to electrical apparatuses subjected to alternating voltage (AC). In fact, partial discharge phenomena in AC have also long been the subject of scientific studies. On the contrary, partial discharge phenomena in the presence of direct voltage (DC) are almost neglected both in the literature and in the current practice of diagnostic activities. In fact, since it is not possible in DC to establish a time correlation between discharge events and profile of the power supply voltage (and hence of the electrical field that generates them), it is extremely complex to separate the discharge phenomenon from the noise and from other disturbance phenomena. Moreover, it should be noted that the correlation between discharge events and phase of the electrical field that generates the discharges is a central element also in systems for interpreting the acquired discharge phenomena, i.e. for associating a discharge activity to a defect of an insulating system to be assessed. Identification is a fundamental step to correctly define the reliability of the component studied, because the different types of defects often produce degradation effects that are highly different from each other.
However, electrical apparatuses subjected to direct voltage are not at all free from partial discharge phenomena, although in DC insulating systems the repetition rate of the discharges is typically lower than in AC insulating systems.
Therefore, if it were possible to separate the noise and the disturbances from the partial discharge phenomena and to distinguish the different types of phenomena present in an apparatus, being able to measure and monitor the activity of partial discharges also in electric apparatus operating in DC, that would be a useful diagnostic tool.
In this regard, diagnostic techniques in use provide for conducting measurements of partial discharges on DC operating electric apparatus subjecting said apparatuses to an alternating voltage (i.e. conducting the measurements of partial discharges in AC), to have available a correlation of the discharges with the phase of the voltage.
For this purpose, methods for measuring partial discharges are known which provide for powering the apparatuses (e.g. electrical cables) with an alternating voltage at 0.1 Hz or with resonating systems.
However, the results obtained with said methods have little reliability, because the electrical conditions created specifically during the measurements by applying an alternating voltage do not match the actual conditions of operation. For example, since the profile of the electric field in a DC powered cable is completely different from the same cable powered in AC, it is possible that during the measurements conducted with said methods (in AC), discharge phenomena may occur that in operating conditions (in DC) would not have developed; or, alternating voltage may highlight as particularly significant a discharge phenomen that is relatively harmless in operating conditions, neglecting instead other discharge phenomena that instead, in operating conditions, are particularly dangerous.